It is currently possible to set these field by checking the recurring payment button, setting the fields, saving the membership level, unchecking the recurring payment button, and saving again, but this is not intuitive for users.
Possible fixes include making that field always visible or simply not giving a grace period for non-recurring levels.
Non-recurring memberships give a grace period here for members with a prior successful order but a current order that is pending (https://github.com/strangerstudios/pmpro-pay-by-check/blob/aa5a4bf390187b41727c49b5d87382d91d723dae/pmpro-pay-by-check.php#L466-L470), but it is not intuitive how to set this field.
Here is the field that should get that information when editing a membership level: https://github.com/strangerstudios/pmpro-pay-by-check/blob/aa5a4bf390187b41727c49b5d87382d91d723dae/pmpro-pay-by-check.php#L72-L77
And the JS that hides that field for non-recurring memberships: https://github.com/strangerstudios/pmpro-pay-by-check/blob/dev/js/pmpro-pay-by-check-admin.js
It is currently possible to set these field by checking the recurring payment button, setting the fields, saving the membership level, unchecking the recurring payment button, and saving again, but this is not intuitive for users.
Possible fixes include making that field always visible or simply not giving a grace period for non-recurring levels.